The present invention relates generally to electrically isolated circuits and more specifically to thermal couplers between electrically isolated integrated circuits.
An electrical isolation boundary is one which does not have direct electrical connections across it but which still allows energy or information transfer across it. They are generally used for safety and interference reasons. Typical examples would include power supplies which run off the wall plug, but have outputs which provide power that is isolated from the wall plug itself. Another example is interfaces to the telephone system. Voice, data, and supervisory functions must be transmitted between the telephone lines and telephone or computing equipment while maintaining a 1000V.sub.RMS electrical isolation between them (mandatory by the FCC Part 68 regulations).
There are several methods used to accomplish this type of transmission. Nearly all of them involve conversion of electricity to some other form of energy which is transmitted across the boundary and then reconverted back to an electrical signal. Two very common ones are optoisolators and transformers.
Optoisolators consist of a light emitter and detector that are generally sealed together in a light-tight package. The emitter's light intensity varies according to the amplitude of the electrical drive applied to it and the detector provides an electrical output whose amplitude depends on the light intensity impinging on it. The emitter and detector are manufactured as two separate objects and are then brought together, separated by a transparent or translucent insulating material, such as plastic.
Transformers convert the incoming electrical signal to magnetic energy by means of a coil. A second coil is then brought into the magnetic field by means of physical proximity or by directing the field using a magnetic material or "core". The second coil will produce an electrical output in response to a changing magnetic field intensity.
These methods present some drawbacks in that they cannot be integrated into a chip without using very expensive processing techniques. As a result, they are generally only available in discrete form and thus present economic and volumetric penalties to system designers. The prior arts attempt to integrate these couplers is discussed in "High Voltage Integrated Circuits for Telecommunications" by Tatsuya Kamei;, Extended Abstract of 1981 IEDM.
Thus an object of the present invention is to provide economic coupler integrated with the electrically isolated regions.
Another object of the present invention is to provide a thermal coupler which is integrated with electrically isolated circuits.
A still further object of the present invention is to provide a thermal coupler which is insensitive to ambient temperatures.
A still even further object of the present invention is to provide a thermal coupler which accurately reproduces the transmitted signal.
A still further object of the present invention is to provide a coupling system for electrically isolated circuits capable of coupling low and high frequency transmission signals.
These and other objects of the invention are attained by an electro-thermal isolator having a thermal transmitter in a first dielectrically isolated region generating a thermal signal from an electrical input signal and two thermal receivers in second and third electrically isolated regions for receiving the transmitted thermal signal and generating an electrical output signal. One of the electric output signals is provided to an error detection circuit for comparison with the desired signal and adjusts the generation of the thermally transmitted signal. Each thermal receiver includes a pair of thermally responsive devices both of which are responsive to ambient temperature and one of which is responsive to the transmitted thermal signal and the other being substantially unresponsive to thermally transmitted signal. The thermally responsive devices are connected to a comparator which provides a thermally compensated electrical output signal. To provide total isolation, each of the dielectrically isolated regions include separate power sources.
In an interface integrated circuit, the electro-thermal isolators are used to couple signals between dielectrically isolated ports or sections of the interface circuits and high voltage capacitors are also used to interconnect portions of the circuit in each of the sections. The thermal means couples low frequency signals wherein the capacitors couple high frequency signals. An oscillator provides a carrier signal which is modulated with an input signal to be transmitted capacitively. The two couplers are compatible with isolation above 1,000 volts. The interface circuit may be used in a subscriber-line interface circuit in telephone systems. The voice signal is transmitted via the capacitive coupling whereas control signals are transmitted via the thermal couplers.
These and other objects of the invention will become apparent from the following detailed description when considered with the accompanying drawings.